HYDRATION OF IONS 255 



by the water molecules when they are reoriented from their water structure. 

 For the K+ ion the three terms were calculated to be: 



(p = 65.2 + 40 - 28 = 77.2 kcal/mole 

 which is quite close to the experimental value. 



Hydration Differences between Cations and Anions 



There has been considerable confusion about the hydration of anions; 

 in fact, it was believed for some time that only cations were appreciably 

 hydrated. However, the hydration energies of anions (Table 6-17) are 

 greater than for cations of comparable radii; for example, K"^ and F~ are 

 approximately the same size and yet the hydration energy of the latter is 

 — 47 kcal/mole greater. The reason for this difference is that the dipole 

 center of water can approach an anion more closely than a cation. The 

 distance from the nucleus of a K"^ ion to the water dipole center is 3.16 A 

 whereas an anion of the same size would approach to within 2.55 A, using 

 the accepted ionic radius and configuration of the water molecule. It might 

 be asked why the primary hydration number of anions is usually given as 

 less than for cations. In fact, the significance of a primary hydration of 

 one or two might be questioned. The fact that the hydration energies of 

 Cr, Br~, and I~ are of the same magnitude as K"*", as are the entropy 

 changes, would indicate that there is about the same degree of interaction 

 with the water molecules and the same disruption of water structure. The 

 larger anionic radius is balanced by the closer approach to the water dipole; 

 in other words, a Br~ ion would have about the same interaction distance 

 with water as a K"^ ion, and a similar primary hydration of six would be 

 expected. In the case of the small F~ ion it is possible that steric factors 

 allow only four water molecules to surround the ion, but for the larger 

 anions this is not a restriction. It may well be that such anions should be 

 considered as having as complete a hydration layer as the cations, since 

 the repulsion energy (dipole-dipole) of the bound water is relatively small 

 compared to the ion-dipole energy. It is likely that the R— COO" group 

 is hydrated but due to its asymmetry it is possible there is a primary 

 hydration of only three. The R — NH3"^ group probably also has a primary 

 hydration of three. These values will be accepted in future calculations as 

 the most reasonable. 



The Structure of Water and the Effects of Solutes on this Structure 



The interactions of molecules in aqueous solution involve not only the 

 hydration water associated with ions or ionic groups but also the structure 

 of water in the bulk phase and surrounding nonpolar molecules or parts 



